To adjust speed and reduce gasoline consumption, every means of transportation nowadays is equipped with a gear shift mechanism. A conventional gear shift mechanism transfers power with a gear train or with a gear train and an oil duct. However, the gear train or the combination of the gear train and the oil duct is structurally intricate and bulky, incurs much transmission loss, and tends to jerk while shifting gear. Therefore, a stepless gear shift mechanism characterized by two grooved wheels operating in conjunction with a V-shaped belt is developed. However, the stepless gear shift mechanism has a disadvantage, namely large volume of the grooved wheels and the V-shaped belt. Accordingly, the present invention aims to disclose a linear gear shift power transfer mechanism which is structurally simple and compact, incurs little transmission loss, and never jerks while shifting gear.